Ultrasounds have many applications in present-day technology in physical and chemical processes. Some general references are:
1) K. S. Suslick, Sonochemistry, Science, 247, pp. 1439-1445 (23 Mar. 1990);
2) W. E. Buhro et al., Material Science Eng., A204, pp. 193-196 (1995);
3) K. S. Suslick et al., J. Am. Chem. Soc., 105, pp. 5781-5785 (1983);
4) Telesonic Co., Products Bulletin.
There are several types of ultrasonic reactors. One of them is the loop reactor, described e.g. in D. Martin and A. D. Ward, Reactor Design for Sonochemical Engineering, Trans IChemE, Vol. 17, Part A, May 1992, 29, 3. Inside this reactor, a liquid which is to be subjected to ultrasound treatment, is caused to flow in a closed loop formed by a vessel provided with a stirrer and by a conduit in which the ultrasound generator is housed.
Also, several transducers may be placed around an elongated enclosure, as in U.S. Pat. No. 5,658,534 and U.S. Pat. No. 6,079,508.
This invention relates to a type of reactor in which the reaction occurs in a localized space filled with a material that is generally a liquid phase, which may contain solid particles. By the term “reaction” is meant herein whatever phenomenon is caused or facilitated by the ultrasound radiation, viz. not necessarily a chemical phenomenon, but a physical one or a combination of the two, as well. A reactor of this type is coupled to a transducer, wherein an oscillating, generally alternating, magnetic field is generated by an oscillating, generally alternating, current—hereinafter called “the exciting current.” The reactor contains a material to be treated by ultrasound, which will be called hereinafter “reaction material”. The reaction material generally comprises a liquid phase and fills the process chamber.
The transducers of ultrasound devices can be of various types. Most common transducers are piezo-electric ones. Therein, the generator of the ultrasound typically consists of a piezo-electric element, often of the sandwich type, coupled with a horn having a generally circular emitting face. Piezo-electric transducers, however, have a maximum power of about 2 kW and a low maximum oscillation amplitude dictated by the fragility of piezo-electric elements, which tend to break under prolonged working load. They are also not reliable compared to magnetostrictive transducers, to be described hereinafter, because their amplitude drifts with operation, causes breakdowns and lower energy output and has to be manually corrected. Similar properties are also possessed by electrostrictive materials polarized by high electrostatic fields.
Another type of transducer is that based on the use of a magnetostrictive material, viz. a material that changes dimensions when placed in a magnetic field, and conversely, changes the magnetic field within and around it when stressed. When a magnetostrictive material is subjected to a variable magnetic field, the material will change dimensions with the same frequency with which the magnetic field changes.
Magnetostrictive materials, to be quite suitable, must present a sufficiently large magnetic stricture at the temperature at which the ultrasonic reactor is intended to be used. To achieve this, proposals have been made to use special magnetostrictive materials, for example, alloys containing rare earth materials: see, e.g., U.S. Pat. Nos. 4,308,474, 4,378,258 and 4,763,030. Such alloys are expensive, and, in spite of their better elastic properties, they suffer major drawbacks, one of them being that they will break if subjected to relatively high power, e.g. 5 Kw, and at lower levels of power they do not transduce enough electromagnetic energy to acoustic energy.
A magnetostrictive transducer must comprise a magnetostrictive element, e.g. a rod or another elongated element, located in a space in which an oscillating magnetic field is produced. In its simplest form, such a transducer would comprise a nucleus of magnetostrictive elements and a coil disposed around said element and connected to a generator of oscillating electric current. However, different forms of transducers can be devised to satisfy particular requirements: for instance, U.S. Pat. No. 4,158,368 discloses a toroidal-shaped core of magnetic metal, about which a coil is wound, which toroid defines with its ends an air gap in which a magnetostrictive rod is located.
The magnetostrictive transducer transduces the electromagnetic power it receives into ultrasonic power, which it transmits to an irradiating device—the wave guide or horn. It will be said hereinafter that the horn irradiates the ultrasound into a process chamber, but no limitation is intended by said expression, which is used only for the sake of brevity. Generally, the horns of the prior art have a slim frusto-conical shape or a stepped or exponential shape. In every case, they concentrate the ultrasonic vibrations and irradiate them from their tip, which is generally circular and anyway of reduced dimensions. The ultrasonic waves have therefore a high intensity only at the tip of the horn and spread out from it in a conical configuration, so that they reach only a part of the process chamber and at any point of said chamber their intensity is reduced, generally proportionally to the square of the distance from the horn tip. At their region of maximum intensity various phenomena occur, including heating, cavitation, evaporation, and so on, which absorb and waste a large portion of the ultrasound energy, resulting in a limited efficiency, which is generally in the order of 20-30%. Additionally, some desired phenomena that are produced by the high energy density at the tip of the horn may become reversed at a distance from said tip: for instance, if it is desired to fragment solid particles, contained in a liquid phase, into smaller ones, such smaller particles may be produced at the tip of the horn, but then migrate through the liquid phase and coalesce to some extent at a distance from said tip, so that the particles finally obtained are not as small as desired.
Material treatment of various materials with high intensity ultrasound has recently achieved a significant role in different industries. Ultrasound treatment has been applied to particle dispersing, emulsification, mixing and dissolving of various materials. Other types of processes suitable with ultrasonic radiation are disclosed in U.S. Pat. Nos. 4,131,238, 4,556,467, 5,520,717, 6,035,897, 6,066,328 and 6,168,762.
U.S. Pat. No. 4,071,225 discloses an apparatus for material treatment by the application of ultrasonic longitudinal pressure oscillations, consisting of an enclosure for material to be treated therein having an interior with two closely-spaced walls at least one of which is made to oscillate at ultrasonic frequencies. The spacing between the walls is such that the pressure oscillations produced at the oscillating wall are reflected by the other wall before they are attenuated to a negligible value, thus producing waves with periodically changing frequencies and resulting in thorough dispersion of fine particles in the liquid vehicle. Intense cavitation appears near the reactor walls of this apparatus and therefore interferes with the transfer of ultrasonic energy to the working volume.
U.S. Pat. No. 6,079,508 discloses an ultrasonic processor, which comprises a hollow elongated member with a plurality of transducers fixed to the exterior of the elongated member and a control means for regulating the frequency of the ultrasonic waves produced by the transducers. Although the processor focuses ultrasonic energy onto the center of the reactor, so that cavitation disperses from the center to the periphery of the reactor, thereby protecting the reactor from cavitation damage, the processor suffers from several disadvantages.
Firstly, the transducers are preferably piezoelectric elements which are produced from a ceramic material, and these types of transducers tend to exhibit flexural failure upon exposure to high oscillating amplitudes. Piezoelectric transducers do not facilitate automatic control of the resonance frequency. In piezoelectric transducers a direct relationship between current and magnetic field is non-existent, and therefore the resonance frequency cannot be automatically adjusted in response to changes in the system during operation. Therefore if the piezoelectric material cannot withstand the oscillation amplitude at which it is vibrating, resulting in cracking or fracture due to the stress acting thereon, the transducers are liable to fail without warning.
Secondly, the affixing of the transducers by bonding or by soldering induces a high intensity of ultrasonic energy at those surfaces on the reactor to which the transducers are affixed. This high intensity results in cavitation to those surfaces on the reactor surface, causing pitting to the irradiator and shortening its life. Thirdly, piezoelectric transducers require a high level of operating voltage due to their relatively high resistivity so that a predetermined amount of current may pass therethrough.
Co-pending International Publication No. WO 03/012800 by the Applicant discloses an ultrasound device for the production of material having dimensions in the order of nanometers, comprising a magnetostrictive transducer and a hollow horn which transmits ultrasonic radiation in a substantially uniform manner throughout a reaction chamber. In response to ultrasonic vibrations, the walls of the horn oscillate elastically and produce alternate compression and decompression. It is desired to provide a reactor in which the irradiator does not oscillate elastically. It is also desired to provide a reactor for effecting other processes, in addition to the production of nano-products.
It is a purpose of this invention, therefore, to provide an ultrasound device that is free from the drawbacks of the prior art ultrasound devices.
It is another purpose of the invention to provide such an ultrasound device which has a higher power than the prior art devices.
It is another purpose of the invention to provide an ultrasound device wherein the transducer is of a new design, namely, a magnetostrictive annular ring.
It is an additional purpose of this invention to provide an ultrasonic device in which intense cavitation does not appear near the reactor walls.
It is a further purpose of the invention to provide such an ultrasound device comprising a transducer that is durable and has a high oscillation amplitude, up to 45 microns.
It is a further purpose of the invention to provide such an ultrasound device comprising a transducer that is not of the piezoelectric type.
It is a still further purpose of this invention to provide a reactor for emulsification, particle dispersion and deagglomeration.
It is a still further purpose of this invention to provide a reactor that is capable of effecting speedy ultra-fine grinding.
It is a still further purpose of this invention to provide means for the acceleration of reactions.
Other objects and advantages of the invention will become apparent as the description proceeds.